It is well known that polymerization of latex monomers via conventional emulsification processes results in smaller particle size rubber (latex) emulsions. The process involves introducing an oil (surfactant) into water which results in the production of micelles or bilayers. Ethylenically-substituted monomers migrate into the interior portions of these micelles or bilayers. The monomers may subsequently undergo radical polymerization to produce nanometer size particles that are stabilized as emulsions by the associated surfactants.
Traditionally, it is deemed desirable to have larger rubber particles for use in the production of resins to achieve enhanced impact strength, among other desirable properties in a final product. Hence, processes were developed to “agglomerate” smaller rubber particles into larger particles for use in a variety of applications. Typical latex emulsions having small rubber particles may be agglomerated to larger rubber particles using a number of different processes, e.g., freeze, chemical, pressure, and mechanical agglomeration.
Known chemical agglomeration processes involve the addition of inorganic and organic acids or anhydrides to facilitate agglomeration of small particles into larger particles, such as disclosed in U.S. Pat. Nos. 3,551,370 and 3,558,541, followed by stabilizing with an emulsifying agent. Generally, chemical agglomeration processes start with a latex that is formulated with a surfactant that coats the rubber particles to keep them dispersed. These chemical agglomeration processes involve pH reduction and application of heat, pressure and/or agitation. The reduction in pH results in destabilization of the surfactant coating from the small rubber particles allowing them to agglomerate to form larger rubber particles. Thereafter, a base is added to regenerate the surfactant, which then associates with or coats the larger agglomerated particles to stabilize the emulsion.
Other prior art agglomeration processes are reflected, for example, in U.S. Pat. No. 5,468,788, which is directed to a chemical process for agglomerating small rubber particles in an aqueous latex to form large rubber particles by addition of a water soluble organic acid and a water soluble organic anhydride to the rubber particles prior to agglomeration.
U.S. Pat. No. 5,847,050 is directed to a two-step, combined chemical and mechanical process for agglomeration of latex particles and formation of a graft copolymer using the enlarged particle latex. Agglomeration is stated to be caused mainly by Brownian coagulation and shear agglomeration through agitation wherein particles collide with each other. Agitation is conducted at a specific number of rotations represented by a mathematical relationship between the shape, size, and paddle type of the agitating element. Several agitation conditions are disclosed.
U.S. Pat. No. 6,080,803 is directed to a process for preparing coarse polymer dispersions comprising at least one conjugated diene monomer and at least one ethylenically unsaturated comonomer polymerized by free-radical aqueous emulsion polymerization in the presence of an emulsifier with or without other additives to form a fine polymer dispersion, which is thereafter agglomerated using a combination of chemical and pressure agglomeration. Chemical agglomeration is accomplished through addition of water-soluble or water-dispersible polymers to the fine polymer dispersion, followed by subjecting the mixture to high-pressure agglomeration through a homogenizer.
Traditional chemical agglomeration processes have disadvantages. It is a complex task to keep the latex stable and obtain larger particles without generating coagulum (masses of rubber). Coagulum is often formed due to process conditions involving, among other things, low pH, elevated temperature, agitation and additives. Coagulum can adversely affect the physical and mechanical properties of the final resin product or create obstacles in their manufacture. In addition, traditional agglomeration processes are often difficult to control in terms of producing particle sizes within specified ranges and distributions for certain applications. While many prior art processes result in larger particles, the particle size achieved is often not what is desired or particle size enlargement takes a very long time to achieve, which is not conducive to scale-up or commercial application. Prior art processes may also not be reproducible given the complexities of managing and maintaining process conditions.
A reproducible, controlled process for agglomeration of smaller rubber particles into larger rubber particles has been discovered that comprises using certain surfactants and adjustment of process conditions, such as pH and time, to permit the production of rubber particles having average particle sizes within a specified size and distribution range, with little or no waste, i.e., production of coagulum. The inventive process provides for agglomeration at a lower pH than that used in traditional processes through the use of specific organic or inorganic acids. The inventive process also provides for the use of a novel surfactant system consisting essentially of two different surfactants to stabilize rubber particles as they agglomerate. Both surfactants must be present in the small particle latex prior to pH reduction. With the reduction of pH, one surfactant is converted from its salt form to an acid form, leaving a second, dispersant surfactant, which is highly stable at low pH, to stabilize the rubber particles throughout the entire agglomeration process, resulting in the formation of less coagulum. At the end of the process, base is added, and the other surfactant is regenerated to facilitate stabilization of the larger rubber particles. Significantly, the inventive process proceeds without the temperature elevation, pressure and agitation requirements of the prior art.
It is an object of the invention to provide a process for the agglomeration of particles of synthetic latex, which controls particle size growth within a specified range and distribution.
It is a further object of the invention to provide a synthetic latex precursor formulation for use in agglomeration processes, comprising a latex in combination with select surfactants.
Still another object of the invention is to provide an agglomerated latex for use in a variety of products, wherein the particle size and distribution is tailored for specific applications through adjustment of process parameters.
Other objects of the invention will be apparent to one skilled in the art.